The present invention generally relates to an optical information recording and reproducing and more particularly, to a track skipping detection and control arrangement for an optical recording and reproducing disc formed with guide tracks for use in an optical information recording and reproducing apparatus.
For the optical information recording and reproducing apparatus as referred to above, there has conventionally been proposed, for example, an arrangement in which, with an optical disc painted or deposited with a photosensitive material being kept rotating, a light beam such as a laser beam or the like, which is converged into a very small diameter spot of less than 1 micrometer, is irradiated onto the optical disc, and by modulating the output intensity of light through recording signals, information such as video signals and digital signals may be recorded on the optical disc in real time as phase variations by concave-convex portions or optical characteristic variations such as variations in refractive index, light reflectance or transmittance, etc., while the information thus recorded may be reproduced by detecting the optical characteristic variations as referred to above.
Based on the arrangement as described above, there may be considered an optical information recording and reproducing apparatus in which guide tracks for guiding tracks to be recorded are preliminarily provided in the form of a concentric circle or in a spiral configuration due to requirements for a higher density, partial writing-in or erasing in a dispersed manner, information is recorded on the predetermined track by applying tracking control so as to follow said guide track, or such information is reproduced from said track.
The guide tracks formed in the optical disc should properly be, for example, in the concave-convex groove-like structure. The information is recorded on a recording medium such as an amorphous metal deposited on the optical disc provided with the guide tracks, and is accumulated in the form of hole formation by the evaporation of the recording medium or local blackening thereof.
The identification of the guide tracks is effected through deviation of light intensity distribution at opposite sides in a direction of the guide track by a far visual field pattern of a laser beam as reflected by the guide track. The deviation as referred to above is subjected to a photo-electric conversion by a photo-detector having two light receiving portions arranged to be parallel to the tangential direction of said guide track so as to be applied to a tracking control means.
For the optical disc having the guide tracks as referred to above, there may be conceived a structure as in FIG. 1, which shows a fragmentary perspective view of an optical recording and reproducing disc 1 to which the present invention may be applied. The optical recording and reproducing disc 1 includes a substrate B, grooves each having a width W, a depth D and a pitch P and formed on one surface R of the substrate B to constitute thereon guide tracks 10a, 10b, 10c, 10d and 10e in a concentric or spiral configuration which are separated from each other by flat portions 11a, 11b, 11c, 11d and 11e provided therebetween, and a layer of photosensitive recording material deposited on the surface R to form a recording layer 12 thereon. A very fine beam of light forming a spot is irradiated onto the disc 1, for example, from the side of the substrate B so as to be focused on the surface R for recording necessary information thereon. For recording and reproduction, the tracking control is applied so that the beam of light forming a spot is provided on the groove-like guide tracks 10a-10e. During the recording, light energy of the beam of light projected onto the groove-like guide tracks 10 is increased by increasing the light output of a light source (not particularly shown) for sensitization of the recording material applied onto the guide tracks 10a-10e, with a result that the light reflectance of the recorded portion on the groove-like guide tracks 10a-10e is varied, and if the above variation of the reflectance is detected through the employment of a beam of light spot with a light output smaller than that used in the recording, the recorded signal may be reproduced. The state where the recording material on the guide tracks 10a-10e is sensitized during the recording is shown at 13, whereat the recording material is blackened, with an increase of the light reflectance. For specific values of the width W, pitch P and depth D of each of the guide tracks 10a to 10e, there may be selected, for example, 0.6 micrometer for the width W, 1.6 micrometer for the pitch P, and 100 .lambda. for the depth D, (the light path length being 1/8 of the wavelength of the laser light source referred to earlier).
Referring to FIG. 2, there is shown a diagram illustrating an optical disc 1 and a scanning locus of the light beam upon occurrence of the track skipping.
As shown in FIG. 2, the optical disc 1 has address signals preliminarily cut in track address areas 2A and 2B by concave and convex portions of the grooves so as to be used for retrieval and identification of the required tracks. Normally, the track address area as referred to above is provided in one or two positions per one track. In other words, in the case where one track is subjected to reproduction through one rotation, the present track addresses may be checked only two times per rotation.
As an order for recording information on the optical disc 1, for example, as shown in FIG. 2, the present track 3 is retrieved, and after reproduction of the track address 2A, the recording mode is started at a recording starting point 7. More specifically, from the point 7, the laser light source is set to a power required for the sensitization of the recording medium, and the light beam is modulated according to the information recording signal. During the track recording, it is possible that the tracking servo deviates from the present track 3, and causes a track skipping while traversing or crossing other tracks, although this may not take place so frequently. As possible causes of the above inconvenience, there may be considered deformation of the grooves on the disc 1, flaws on the surface thereof, or unstability of the tracking servo at the starting of the recording when the recording is made by dividing into sectors. Moreover, such track skipping also takes place when external vibrations, impacts, etc. are applied to an optical head (not particularly shown).
Even when the track skipping has occurred during recording as described above, it can not be detected until the address signal 2B is reproduced as described earlier. In other words, the track skipping is not detected for a section 6, and the information is undesirably recorded on a track 5 after the skipping, with a mis-identification thereof as the present track 3. What is more disadvantageous is that, if a track skipping as shown at 8 takes place, the recording is effected by crossing the tracks 4, and if information has been already recorded on the tracks 4, the so-called double writing-in is effected, thus resulting in a spoiling of the information in the tracks 4. Upon consideration of a case where the skipping traverses still more tracks, all the information of the tracks thus traversed are subjected to the double writing-in, with a consequent loss of the information. Furthermore, any tracks 4 thus traversed, even the tracks 4 which are not recorded with information as yet, become incapable of recording.
Since the damage due to the track skipping is very large as described so far, it is necessary to immediately stop the recording mode when the track skipping takes place during the recording.